Conventionally, an optical device comprising a deflective reflecting facet and two stationary plane mirrors has been proposed in Japanese Unexamined Patent Publication No. S51-6563. The two stationary plane mirrors are disposed to face the deflective reflecting facet which can be rotated or swivelled about its rotational axis. An incident light beam deflected by the deflective reflecting facet is reflected by the two stationary plane mirrors sequentially, and is incident on the deflective reflecting facet again, thereby correcting displacement in exit direction of deflected optical beam due to misalignment of the rotational axis of the deflective reflecting facet or misalignment of each deflective reflecting facet itself.
Further, a light deflective optical system comprising a deflective reflecting facet and two stationary plane mirrors has been proposed in Japanese Unexamined Patent Publication No. S61-7818 (U.S. Pat. No. 4,796,965). The two stationary plane mirrors are disposed to face the deflective reflecting facet which can be revolved or swivelled about its rotational axis and the two stationary plane mirrors are arranged such that the edge lines of the two mirrors are on a plane perpendicular to said rotational axis of the deflective reflecting facet. An incident light beam is incident on the deflective reflecting facet through a space between the two stationary plane mirrors. The deflected light beam reflected by the deflective reflecting facet is reflected by the two stationary plane mirrors sequentially and is incident on the deflective reflecting facet again so that the deflected light beam reflected is projected to pass through a space between the deflective reflecting facet and the two stationary plane mirrors or between the two stationary plane mirrors, thereby correcting scanning line distortion.
However, since no analytical study has been made and no reference about incident angle of 20° or less has been made in the Japanese Unexamined Patent Publication No. S61-7818, the light deflective optical system cannot properly correct scanning line distortion.
In the optical system in which light beam deflected twice by the same deflective reflecting facet, the trail of the light beam may be curved after the second time deflecting reflection depending on the incident angle of the light beam. Therefore, depending on position of the optical axis direction of an optical member such as a lens of the scanning optical system after deflection, a wider effective range in the sub-scanning direction (perpendicular to the scanning direction) is required. However, it is hard to manufacture an anamorphic lens, to be used in a scanning optical system, having a wide effective range both in the scanning direction and the sub scanning direction. This causes another problem that the degree of freedom of arrangement among a plurality of lenses is reduced.
The narrower the required effective range of optical member such as a lens is, the higher the accuracy of an optical face is easily obtained.
In any event, it is very advantageous if the curvature in scanning line trail is restrained to very small at any position in the optical direction.
By the way, in an optical scanning device like the optical scanning device proposed in Japanese Unexamined Patent Publication No. S51-6563 in which an incident light beam is reflected by even number stationary plane mirrors sequentially so that the light beam is incident twice on the same deflective reflecting facet to deflect the light beam, there is no necessary to provide an optical system for correcting the surface tilt error in the deflective reflecting facet from a viewpoint of compensation of scanning line displacement.
However, the scanning angle must be small because the light beam interferes with the stationary plane mirrors when the light beam is incident on and projected to the deflective reflecting facet at a right angle to the sub-scanning direction. Therefore, the incident or exit angle is required to be an angle not the right angle to the deflective reflecting facet. This may make a difference in exit angle that causes the curvature of scanning line trail.
In addition, a scanning line displacement on an imaging surface may be caused. The amount of the scanning line displacement is a product obtained by multiplying the displacement at the second deflection due to the surface tilt error by a transversal enlargement ratio β of the sub-scanning direction of the scanning optical system.
In case that the value of surface tilt error varies at the beam incident position depending on the rotation of the deflective reflecting facet because the deflective reflecting facet has a twisted or curved portion, there must be differences in exit angle as compared to a case that the deflective reflecting facet has a complete plane, thus causing further curvature in the scanning line trail. In case that the twisting or curving degrees of deflective reflecting facets differ from each other, the exit angles vary every deflective reflecting facet, thus causing scanning line distortion on the imaging surface.